Getting null reference exception when using Admob Reward Ad in unity.

I have implemented Admob reward ad in my unity android game. No errors and I build the apk and run it using Android Studio.
I have successfully shown the video ad, but when I try to close the ad, the app crashes and closes. Th following error is shown in the logcat.

There is no line umber shown. How will we debug this?

E/Unity: NullReferenceException: Object reference not set to an instance of an object
      at Player.Update () [0x00000] in <filename unknown>:0 
    (Filename:  Line: -1)

add line in void start(){

this.rewardBasedVideo = RewardBasedVideoAd.Instance;

hello, i am having a very similar issue. Did you fix this by any chance?